Device-to-Device (D2D) communication is about direct communication between devices. Sometimes a radio access network with access nodes is present and the devices operate within this radio network, but in some scenarios the communication devices itself constitute the radio access network.
There are several basic (potential) motivations for introducing the possibility for D2D communication.
Although the overall communication service can be provided by means of conventional device-to-access-point communication it can sometimes be provided in a better way with direct D2D communication. It could e.g. be so that the communication between two devices could be achieved at higher rate, with shorter latencies, or using less network resources (less capacity of an access node) if the communication is done directly between the devices than via network nodes (access nodes). The later can be seen as an offloading of the network.
A second reason for direct D2D communication is if the service to be provided cannot be supported (within given requirement boundaries) unless direct D2D communication is applied.
A third reason for supporting direct D2D communication is to ensure communication availability even if the network infrastructure for one reason or another is not available. This reason for supporting direct D2D communication is often mentioned e.g. in relation to National Security and Public Safety (NSPS) services but also for traffic safety applications. More generally, one can say D2D communication is used in this scenario to increased robustness and reliability.
Also, sometimes information is only valid or of interest in a local area, in which case it can make sense to rely on D2D communication.
Network-assistance can help in setting up a D2D service, i.e. assist in service and peer discovery and also in establishing security of the D2D link. Furthermore, the interference environment is under control for the network, which enables usage of licensed operator's spectrum (since the communication is under operator's control) whereby there is higher reliability than in unlicensed spectrum. The network can also provide synchronization and can assist in Radio Resource Management (RRM). An application of network assisted D2D is network offloading, where user plane data is exchanged directly between nearby terminals without been sent via a base station in order to reduce the load on the network nodes.
Prior to setting up a D2D communication between two devices, say D1 and D2, the network node does not know whether D1 and D2 is in proximity to each other, so that D2D communication is possible. A typical approach is to allocate a beacon signal to D1 and then require D2 to listen for the beacon and report detection of the beacon signal. There is a risk for interference with other units network communication or D2D communication during beacon signalling, and it can be a capacity waste to allocate, say orthogonal (such that other communication is not interfered) resources for beacon signalling if it turns out that the devices are too far from each other.
US 2010/0279627 discloses an apparatus and a method of switching between a D2D mode and a cellular mode in response to a command from a controller. Upon receiving a control command, both user equipments (UEs) set up for D2D mode communication, switching from cellular mode to D2D mode for direct communication with each other.
US 2010/093364 relates to a base station which schedules device-to-device communication between a UE pair based on interference or path loss measurements. The base station can also schedule the D2D groups on resources with low interference from and to UEs engaged in cellular communication.
US 2012/076086 provides a method of allocating wireless communication capacity in a wireless point-to-point link including obtaining a channel having a bandwidth for use in the wireless point-to-point link, allocating a first portion of the bandwidth for use for transmitting from a first point to a second point of the wireless point-to-point link, and allocating a second portion of the bandwidth for use for transmitting from the second point to the first point of the wireless point-to-point link, in which the bandwidth is asymmetrically assigned between the first portion and the second portion. To provide more capacity in one direction than another, an available spectrum is taken and split it into smaller segments, or sub-bands, for example sub-bands of 7 MHz each, or 3.5 MHz each, and the different sub-bands are allocated asymmetrically over the links. Thus, the document is related to asymmetrical scheduling of frequency resources based on asymmetrically needed capacity.
US 2012/087253 discloses peer-to-peer scheduling in addition to cellular wireless wide area network (WWAN) scheduling, where a wireless device can communicate with another wireless device via peer-to-peer while also communicating with a base station.